1. Field of the Invention
This invention relates to a spark gap arrangement capable of carrying lightning current.
2. Description of Related Art
German published patent application No. DE-OS 39 14 624 discloses a spark gap arrangement which includes two series-connected spark gaps and is capable of carrying lightning current. Each spark gap consists of two electrodes and an insulating layer arranged between the two electrodes, thereby providing a spark-over path is provided between the two electrodes, and the thickness of at least one of the insulating layers in the two spark gaps is different from the thickness of the other insulating layer(s). In this arrangement, the insulating layers with different thicknesses consist of the same material and accordingly have the same specific resistance.
German patents DE-PS 29 34 238 and 29 34 236 also disclose spark gap arrangements which include a plurality of series-connected spark gaps and are capable of carrying lightning current, and in which each spark gap consists of two electrodes and an insulating layer with a spark-over path arranged between the two electrodes. Such arrangements are particularly widely used in low-voltage installations, especially on the input side when the installation is connected to the main power line or grid.
In the arrangement described in DE-PS 29 34 238, for example, the insulating layers are made of a material which, when heated, and in particular when heated by an arc, gives off a gas which pushes or blows the arc to the outside. The insulating material is described as being preferably in the form of a thermoplastic plastic that gives off hydrogen gas (H.sup.2), e.g. polymethylene oxide (PMO). However, this publication does not touch upon the subject matter of the present invention, either by recognizing the problem solved or by proposing a solution, as will become apparent from the description of the invention which follows this description of related art. The arrangement described in DE-PS 29 34 236 similarly attempts to provide improved dissipation of currents, but has the disadvantage of a relative high response voltage, which renders its practical use in certain field installations difficult.
On the other hand, DE-PS 39 14 624 describes an arrangement which attempts to achieve a low response voltage with a high current carrying capacity and direct dissipation of a primary lightning-induced current after a strike, but in practice is only able to achieve relatively small capacity ratios of 1:6. As a result, practical use of this arrangement is limited whenever higher requirements have to be met.
Swiss patent No. CH-PS 449 106 discloses an over-voltage arrester which has a series connection of spark gap stacks and voltage-dependent resistors, in which the spark gap stacks and the voltage-dependent resistors alternate in the series. In this arrangement, every spark gap stack is connected in parallel to a control resistor. The space between the active part of the arrangement and an insulating housing surrounding it is filled with a foam, the pores of which contain an electro-negative gas. The Swiss publication does not give any indication as to the type of control resistors which ensure that a control effect can be obtained, nor does it discuss the control effect itself. Furthermore, the over-voltage arrester according to CH-PS 449 106 is expensive to produce as a result of the indicated series connection and the parallel control resistors, and as a result of the provision of a housing, which also has the disadvantage of requiring a considerable amount of space, which in practice is often not available.
The primary objective of the above-mentioned devices is to provide a spark gap arrangement capable of carrying lightning current in which, after a current pulse reaches a protection level, the energy contained in the current pulse is safely and controllably discharged in a manner which protects connected installations and equipment.
In order to accomplish this objective, the excess current which occurs when the spark gap responds is dissipated in the next zero passage of the current, or must be carried without destruction until interruption by a pre-fuse. This results in contradictory requirements. On the one hand, the response voltage of the spark gap must be as low as possible, which as a rule is obtained by minimizing the distance between the electrodes of the spark gap. On the other hand, for secure dissipation of the short circuit current, it is desirable to provide a voltage drop across the spark gap which is as high as possible, and which can best be realized by a great distance between the electrodes. This in turn, however, increases the response voltage (see above).
Other known measures for dissipating the short circuit current are also disadvantageous. For example, an increase in the field strength of the arc by cooling requires a correspondingly large volume of the spark gap. Also, the series connection of several spark gaps, which is realized in the aforementioned state of the art, causes an undesirable increase in the response voltage of the overall arrangement.